This invention relates to optical assemblies (optical units) for the effective polarization separation of light. The assemblies can be used with, for example, reflective liquid crystal on silicon devices (LCoS devices).
More specifically, the invention relates to polarization separation devices known as polarization beam splitters (also referred to in the art as xe2x80x9cpolarized beam splitters,xe2x80x9d xe2x80x9cpolarizing beam splitters,xe2x80x9d or simply xe2x80x9cPBSsxe2x80x9d) and, in particular, to polarization beam splitters for use in image projection systems which employ one or more reflective, polarization-modulating, imaging devices.
A. Image Projection Systems
Image projection systems are used to form an image of an object, such as a display panel, on a viewing screen. Such systems can be of the front projection or rear projection type, depending on whether the viewer and the object are on the same side of the screen (front projection) or on opposite sides of the screen (rear projection).
FIG. 1 shows in simplified form the basic components of an image projection system 77 for use with a microdisplay imaging device (also known in the art as a xe2x80x9cdigital light valvexe2x80x9d or a xe2x80x9cpixelized imaging devicexe2x80x9d). In this figure, 70 is an illumination system, which comprises a light source 71 and illumination optics 72 which transfer some of the light from the light source towards the screen, 73 is the imaging device, and 74 is a projection lens which forms an enlarged image of the imaging device on viewing screen 75.
For ease of presentation, FIG. 1 shows the components of the system in a linear arrangement. For a reflective imaging device of the type with which the present invention is concerned, the illumination system will be arranged so that light from that system reflects off of the imaging device, i.e., the light impinges on the front of the imaging device as opposed to the back of the device as shown in FIG. 1. Also, as shown in FIGS. 2 and 3, for a reflective imaging device which operates by modulating (changing) the polarization of portions of the illumination light (referred to herein as a xe2x80x9creflective, polarization-modulating, imaging devicexe2x80x9d), a polarization beam splitter (PBS) will be located in front of the imaging device and will receive illumination light 11, e.g., S-polarized light, from the illumination system and will provide imaging light 12, e.g., P-polarized light, to the projection lens.
For front projection systems, the viewer will be on the left side of screen 75 in FIG. 1, while for rear projection systems, the viewer will be on the right side of the screen. For rear projection systems housed in a cabinet, one or more mirrors are often used between the projection lens and the screen to fold the optical path and thus reduce the system""s overall size.
Image projection systems preferably employ a single projection lens which forms an image of: (1) a single imaging device which produces, either sequentially or simultaneously, the red, green, and blue components of the final image; or (2) three imaging devices, one for red light, a second for green light, and a third for blue light. Rather than using one or three imaging devices, some image projection systems have used two or up to six imagers. Also, for certain applications, e.g., large image rear projection systems, multiple projection lenses are used, with each lens and its associated imaging device(s) producing a portion of the overall image.
B. Polarization Beam Splitters
FIG. 2 shows a conventional layout for an image projection system employing a polarization beam splitter 60 of the MacNeille cube type. See, for example, E. Stupp and M. Brennesholtz, xe2x80x9cReflective polarizer technology,xe2x80x9d Projection Displays, 1999, p. 129-133. As shown in this figure, the polarization beam splitter (PBS) consists of two optically cemented right-angle prisms 61 and 62. The diagonal 63 of the splitter has a dielectric coating that reflects S-polarized light and transmits P-polarized light.
As can be seen in FIG. 2, after reflecting off of the diagonal of the MacNeille-type PBS, S-polarized light 14 from the illumination system reaches reflective imaging device 10, e.g., a LCoS device, where it is polarization modulated. The modulated light 15 is P-polarized and thus passes through diagonal 63 and on to the projection lens to form the desired image. Non-modulated light (not shown in FIG. 2), which is still S-polarized, reflects from the diagonal and is returned to the illumination system.
The main problem with using a MacNeille-type PBS in image projection systems is the depolarization of transmitted light that is caused by skew-ray effects. This is a purely geometrical phenomenon and is described in Miyatake, U.S. Pat. No. 5,327,270, which issued on Jul. 5, 1994, and is entitled xe2x80x9cPolarizing Beam Splitter Apparatus and Light Valve Image Projection System.xe2x80x9d
This depolarized light reduces the contrast of the system. In accordance with the Miyatake patent, compensation of the skew-ray depolarization requires an additional quarter-wave plate (i.e., plate 64 in FIG. 2), which adds cost, requires precision alignment, and restricts the range of operating temperatures. Other disclosures of the use of compensating plates in projection systems employing reflective, polarization-modulating, imaging devices can be found in Ootaki, U.S. Pat. No. 5,459,593, Schmidt et al., U.S. Pat. No. 5,576,854, and Bryars, U.S. Pat. No. 5,986,815.
Another type of PBS is the wire grid polarizer. See, for example, Perkins et al., U.S. Pat. No. 6,122,103, which issued on Sep. 19, 2000, and is entitled xe2x80x9cBroadband Wire Grid Polarizer for Visible Spectrum.xe2x80x9d This optical component does not suffer from skew-ray depolarization, and also has a very high polarization extinguish ratio. In addition, the component works over a large temperature range and can withstand a high light intensity. A wire grid polarizer 13a can be used with reflective, polarization-modulating, imaging devices, e.g., LCoS devices, in accordance with the component layouts shown schematically in FIGS. 3A and 3B. Unfortunately, both of these layouts suffer from optical problems.
The optical problem associated with the layout of FIG. 3A is that there is a tilted plano-parallel plate in the imaging optical path. The plate is the glass substrate (thickness greater than 0.5 mm) that supports the wire grid structure. Currently, a technological limitation in the process that creates the wire grid structure makes the use of a thinner substrate difficult. A glass substrate 0.5 mm thick, tilted at 45 degrees creates astigmatism of xe2x88x920.135 mm. See Warren J. Smith, Modern Optical Engineering, 2nd edition, McGraw-Hill, Inc., New York, 1990, page 99. The typical depth of focus of a projection lens used with a LCoS device is +/xe2x88x920.025 mm (for an f-number (FNo) of 2.8). Therefore, the layout of FIG. 3A has unacceptable image quality due to astigmatism that is 2.5-3 times larger than the depth of focus.
In the layout of FIG. 3B, the light passes through the tilted glass substrate in the illumination path, where astigmatism is not critical. In this case, the image quality in the imaging path depends on the flatness of the wire grid substrate. Acceptable image quality requires the surface flatness to be about 1 fringe per inch or better. The best wire grid polarizers available today have a flatness of about 3 fringes per inch. There are also two other problems associated with the layout of FIG. 3B: (1) temperature deformation and (2) wire grid structure protection.
Typically, a LCoS projector is assembled and aligned at room temperature, but the operational temperature in the area of the LCoS (where the wire grid PBS is located) is 45-55 degrees Celsius. This elevated temperature can create deformation of the wire grid substrate, which will degrade the image quality on the screen.
As to the protection problem, the wire grid structure should be protected from environmental dust, humidity, mechanical scratches, etc., which will reduce the polarization properties of the PBS. But any kind of protective window applied in front of the grid structure in the configuration of FIG. 3B will essentially reintroduce a plano-parallel plate into the imaging path, which will create astigmatism as discussed above.
Another known type of PBS is a multi-layer reflective polarizer. See, for example, Jonza, et al., U.S. Pat. No. 5,965,247, the contents of which are incorporated herein by reference. See also Private Line Report on Projection Display, Volume 7, No. 1, Jul. 20, 2001, pages 6-8.
Like wire grid polarizers, multi-layer reflective polarizers fall into the general class of Cartesian polarizers in that the polarization of the separate beams is referenced to the invariant, generally orthogonal, principal axes of the polarizer so that, in contrast with a MacNeille-type PBS, the polarization of the separate beams is substantially independent of the angle of incidence of the beams. See Bruzzone et al., U.S. Pat. No. 6,486,997, the contents of which are incorporated herein by reference.
FIG. 3C schematically shows a layout for using a multi-layer reflective polarizer 13b with a reflective, polarization-modulating, imaging device 10, e.g., a LCoS device. Multi-layer reflective polarizers are relatively thick components and, as shown in FIG. 3C, are tilted at an angle of 45 degrees.
The thickness of this component in combination with the difference in refractive index between the component and the surrounding glass prisms 51 and 52 creates astigmatism, which degrades the display""s image quality. For example, a multi-layer reflective polarizer can have a thickness and index of refraction of 0.25 millimeters and 1.54, respectively, while the index of refraction of prisms 51 and 52, when composed of PBH-56 glass, is approximately 1.85. When tilted at 45xc2x0, such an arrangement creates astigmatism of approximately 0.2 mm. To correct this astigmatism, a plano-parallel plate 50 (astigmatism corrector) having a high index of refraction (e.g., approximately 1.93 for PBH-71 glass) can be used next to the multi-layer reflective polarizer as shown in FIG. 3C. However, the use of such an astigmatism corrector significantly increases the cost of the PBS.
In view of the foregoing, there exists a need in the art for polarization beam splitters which have some and preferably all of the following properties:
(1) the PBS is easy to manufacture and does not require thin or ultra-flat substrates;
(2) the PBS is environmentally protected;
(3) the PBS is not subject to deformation at elevated temperatures; and
(4) the PBS does not introduce substantial levels of astigmatism into the image light.
As an additional fifth property, the optical path through the PBS for imaging light is preferably short so that the projection lens which forms the ultimate image can have a shorter back focal length and thus a simpler and less expensive construction.
To satisfy this need in the art, the invention provides polarization beam splitters for use with reflective, polarization-modulating, imaging panels which have some and preferably all of the above five features.
In particular, in accordance with a first aspect, the invention provides an image projection system (77) comprising:
(I) an illumination system (70) which produces polarized illumination light (11) having a first polarization direction (preferably, S-polarization);
(II) a reflective imaging device (10) which receives polarized illumination light (11) and produces modulated reflected light by changing the polarization direction of selected portions of the received light to a second polarization direction (preferably, P-polarization);
(III) a projection lens (74); and
(IV) a prism assembly (33) which comprises an input prism (20), an output prism (30), and a polarizer (13) between the input (20) and output (30) prisms,
wherein:
(A) the input prism (20) comprises:
(i) a first surface (21) which receives polarized illumination light (11) from the illumination system (70);
(ii) a second surface (22) which provides polarized illumination light (11) to the imaging device (10) and receives modulated reflected light from the imaging device (10); and
(iii) a third surface (23) which faces the output prism (30);
(B) the output prism (30) comprises:
(i) a first surface (31) which faces the input prism (20) and is parallel to the third surface (23) of the input prism (20); and
(ii) a second surface (32) which provides light to the projection lens (74) to form a projected image; and
(C) the polarizer (13):
(i) is between the third surface (23) of the input prism (20) and the first surface (31) of the output prism (30); and
(ii) reflects light having the first polarization direction and transmits light having the second polarization direction;
wherein the polarized illumination light (11) has an optical path which comprises:
(i) inward transmission through the first surface (21) of the input prism (20);
(ii) total internal reflection at the second surface (22) of the input prism (20);
(iii) outward transmission through the third surface (23) of the input prism (20);
(iv) reflection from the polarizer (13);
(v) inward transmission through the third surface (23) of the input prism (20); and
(vi) outward transmission through the second surface (22) of the input prism (20).
In accordance with a second aspect, the invention provides a prism assembly (33) which comprises an input prism (20), an output prism (30), and a polarizer (13) between the input (20) and output (30) prisms, where:
(A) the input prism (20) comprises:
(i) a first surface (21) which is configured and arranged to receive polarized illumination light (11) from an illumination system (70);
(ii) a second surface (22) which is configured and arranged to provide polarized illumination light (11) to an imaging device (10) and to receive modulated reflected light from the imaging device (10); and
(iii) a third surface (23) which faces the output prism (30);
(B) the output prism (30) comprises:
(i) a first surface (31) which faces the input prism (20) and is parallel to the third surface (23) of the input prism (20); and
(ii) a second surface (32) which is configured and arranged to provide light to a projection lens (74) to form a projected image; and
(C) the polarizer (13):
(i) is between the third surface (23) of the input prism (20) and the first surface (31) of the output prism (30); and
(ii) reflects light having a first polarization direction and transmits light having a second polarization direction;
wherein the polarized illumination light (11) has an optical path which comprises:
(i) inward transmission through the first surface (21) of the input prism (20);
(ii) total internal reflection at the second surface (22) of the input prism (20);
(iii) outward transmission through the third surface (23) of the input prism (20);
(iv) reflection from the polarizer (13);
(v) inward transmission through the third surface (23) of the input prism (20); and
(vi) outward transmission through the second surface (22) of the input prism (20).
In accordance with a third aspect, the invention provides a method for producing an image using a polarizer (13) which reflects light of a first polarization (preferably, S-polarization) and transmits light of a second polarization (preferably P-polarization), said method comprising in order:
(1) providing polarized illumination light (11) having a first polarization direction (preferably, S-polarization);
(2) introducing the polarized illumination light into a prism (20) having a plurality of surfaces (21,22,23);
(3) changing the direction of the polarized illumination light through total internal reflection at one of the prism""s surfaces (22);
(4) reflecting the polarized illumination light from the polarizer (13);
(5) modulating the polarization of the polarized illumination light at a reflective imaging device (10) by changing the polarization of selected portions of that light to the second polarization, said selected portions comprising the light which forms the image; and
(6) transmitting the selected portions through the polarizer (13) and to a projection lens (74) to form the image.
In accordance with a fourth aspect, the invention provides a method for producing an image using a polarizer (13) which reflects light of a first polarization (preferably, S-polarization) and transmits light of a second polarization (P-polarization), said method comprising in order:
(1) providing polarized illumination light having the second polarization direction (e.g., imaging light 12 propagated in the opposite direction in FIG. 6);
(2) transmitting the polarized illumination light through the polarizer (13);
(3) modulating the polarization of the polarized illumination light at a reflective imaging device (10) by changing the polarization of selected portions of that light to the first polarization, said selected portions comprising the light which forms the image;
(4) reflecting the selected portions having the first polarization from the polarizer (13) to form image light (e.g., illumination light 11 propagated in the opposite direction in FIG. 6);
(5) introducing the image light into a prism (20) having a plurality of surfaces (21,22,23);
(6) changing the direction of the image light through total internal reflection at one of the prism""s surfaces (22); and
(7) transmitting the image light to a projection lens (74) to form the image.
In accordance with each of these aspects of the invention, the polarizer is preferably either a wire grid polarizer (13a) or a multi-layer reflective polarizer (13b).
The reference numbers used in the above summaries of the various aspects of the invention are only for the convenience of the reader and are not intended to and should not be interpreted as limiting the scope of the invention. More generally, it is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention.
Additional features and advantages of the invention are set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification.